1. Field of the Invention
This invention relates to a diesel particulate filter apparatus adapted to collect by a filter an exhaust gas discharged from a diesel engine, and incinerating the same.
2. Description of the Prior Art
The combustion in a diesel engine is based on the heterogeneous mixing carried out by injecting a fuel into high-temperature and high-pressure air. In the heterogeneous mixing, air and a fuel are not uniformly mixed unlike those in a homogeneous gaseous mixture. Therefore, the carbon component in the fuel turns into soot and HC due to the high combustion heat, and they are agglomerated into particulates, which are discharged to the outside to cause the contamination of the environment.
In a conventional exhaust emission control apparatus, a filter is formed to a large area, and the carbon in the exhaust gas from an engine is accumulated on a front surface of the filter, whereby the carbon is collected. In order to incinerate the carbon accumulated on the filter, a heater is provided on a part of the filter, and the accumulated particulates including carbon are incinerated, whereby the filter is regenerated.
Japanese Patent Laid-Open No. 256812/1990 discloses a particulate trap which can be electrically regenerated. This particulate trap comprises a gas permeable support, a plurality of fiber layers of ceramic fiber, and a heater, and the fiber layers and heater are provided in a space extending radially from the gas permeable support. An exhaust gas is introduced into clearances between the fiber layers, and the particulates in the exhaust gas are collected in a position near the heater, the collected particulates being incinerated by supplying an electric current to the heater. The material for the ceramic fibers is selected from continuous molten silica, glass, alumina silica, zirconia silica, alumina chromia, titania, graphite, silicon carbide and alumina boria silica. The ceramic fiber is directly braided on or wound around the surface of the gas permeable support or heater.
It is necessary that the diesel particulate filter be made compact so that it can be mounted on a vehicle conveniently and collect particulates, such as carbon, soot and HC efficiently. In general, the particulates are synthetic products of carbon and hydrocarbon contained in an exhaust gas from a diesel engine, and the size thereof varies from several .mu.m to several ten .mu.m. Although the particulates react with oxygen and are burnt easily, a high temperature is required for the burning thereof. The particulates cannot be burnt completely at the temperature of an exhaust gas alone.
A conventional diesel particulate filter apparatus is provided with two filters of the same size arranged in parallel with a flow of an exhaust gas. When one filter is clogged with collected particulates, the exhaust gas is supplied to the other filter, while the air required for the incineration of particulates is sent to the clogged filter so as to heat the filter and incinerate the collected particulates, whereby the filter is regenerated. Regarding the conventional diesel particulate filter apparatuses, various particulate and smoke eliminating apparatuses have been developed but all of these apparatuses have a complicated construction and a troublesome function, so that they cannot be put to practical use under the present conditions.
In a diesel particulate filter apparatus, the particulates accumulated on a filter are incinerated by supplying an electric current to a heater provided on a surface of the filter. In such an apparatus, it is necessary that the heater has a thermal resistance of not lower than 900.degree. C., and a large resistance temperature coefficient which indicates that a resistance value becomes higher as the temperature increases. For example, when an incineration temperature is increased during the incineration of the particulates accumulated on a filter, electric power continues to be supplied to a heater even if the portion of the heater on which the carbon begins to be burnt by the electric current supplied to the heater comes to have a locally increased temperature. Consequently, the temperature of such a portion of the heater further increases, so that metal wires constituting the heater are burnt and broken.
Accordingly, if an electric current stops flowing to the portion alone of the heater provided on the filter at which the temperature has increased to a high level locally, a local temperature rise in the heater is restrained, and the heater is necessarily maintained at a uniform temperature. However, a material having the above-mentioned characteristics of a heater and a high resistance temperature coefficient, a high volume resistance value and, moreover, a heat resistance and a high corrosion resistance does not exist.